1. Field of the Invention
The present invention relates to techniques for communicating data through a communication channel. More specifically, the present invention relates to a method and an apparatus for increasing a data transfer rate through a communication channel.
2. Related Art
Advances in semiconductor fabrication technology presently make it possible to integrate large-scale systems, including tens of millions of transistors, into a single semiconductor chip. Integrating such large-scale systems onto a single semiconductor chip enables increases in the frequency at which such systems can operate, because signals between system components do not have to cross chip boundaries, and are not subject to lengthy chip-to-chip propagation delays.
However, as the frequency of these systems increases, the communication channels used to transfer data between system components is becoming a bottleneck. This can cause the system to waste time waiting for data to arrive. One solution to this problem is to increase the frequency at which the signal is transmitted on the communication channel to allow more data to be sent through the communication channel per unit time. Unfortunately, the frequency of a signal cannot be increased indefinitely. In a typical lossy communication channel, as the signal frequency increases, the amplitude of the signal decreases. This makes the signal more vulnerable to noise. Differential signaling can be used to somewhat increase the bandwidth of lossy communication channels, but improvements gained through this technique are limited.
Another method to increase bandwidth is to use multi-level signaling, which increases the effective bandwidth by increasing the amount of data transferred in a given time unit. For example, FIG. 1A presents a voltage-versus-time diagram of a four-level signaling scheme. It illustrates four voltage levels, including level 102, level 204, level 106, and level 108. Since there are four distinct voltage levels in this four-level signaling scheme, each voltage level can convey two bits of data. Unfortunately, receivers are not good at distinguishing between multiple voltage levels, especially if there is any noise on the communication channel.
Hence, what is needed is a method and an apparatus for increasing the data transfer rate through a communication channel without the problems described above.